Implantable stimulation devices of the type having electrical circuit components are well known in the medical arts. In one particularly common form, the implantable device comprises a pacemaker unit having an appropriate electrical power supply and related control circuitry for use in electrically stimulating a patient muscle, such as the heart. Such pacemaker units commonly include an hermetically sealed case or housing within which the power supply and control circuitry are protectively encased, in combination with one or more conductive pacemaker leads extending from the housing to the selected muscle structure within the patient. Feedthrough terminals on the pacemaker housing accommodate hermetically sealed passage of electrical conductors to the housing exterior for appropriate connection to the pacemaker lead or leads, typically through the use of so-called connector blocks having set screws or the like for secure lead attachment. The connector blocks and associated feedthrough conductors disposed externally of the pacemaker housing are commonly encased within a sealed head structure, such as an insulative head of cast epoxy or the like.
The commonly used form of welding which has heretofore been satisfactory for making connections in implantable stimulation devices between leads and either electrodes and connectors has been resistance welding which unfortunately is operator dependent with many variables including electrode wear, force, and voltage. The inventors and others have come to recognize that laser welding would be desirable for joining small diameter wire to electrodes and connectors, notwithstanding the fact that resistance welders are less expensive than laser welders.
In many instances, present laser weld design concepts for joining small diameter wire or rod or coiled wire to electrodes and connectors cannot be used to reliably produce a joint. Components made from dissimilar materials having different melting temperatures, normal component fabrication variability, insignificant thermo mass inherent with certain components, imprecise component alignment during assembly, and unlike materials with distinctly different melting temperatures, are major factors that affect the reliability and repeatability of weld connections using conventional design concepts.
Typical of more recent developments in this regard is U.S. Pat. No. 5,458,629, to Baudino et al. In this instance, a ring electrode may be introduced onto an insulated lead so as to form an isodiametric lead construction. The outer layer of insulation forming the lead body is etched or notched, for example, by being laser etched or physically milled to provide a recess in the lead insulation having a depth corresponding to the thickness of the ring electrode intended to be provided at that location. A ring electrode is introduced onto the notched section on the lead in the form of a C-shaped sleeve adaptable to be introduced onto the notched portion of the lead and subsequently formable into a cylindrical shape when closed into position in the notched portion of the lead so that the edges of the C-shaped sleeve are brought to an abutting as opposed to overlapping relationship. A single conductor is brought through the insulation and aligned with a hole in the C-shaped sleeve to be welded to the sleeve, for example, by laser welding. The final affixation procedure involves laser welding the abutting surfaces of the sleeve together, thereby securely forming a ring electrode isodiametrically within the notch on the electrode.